Plating of tin-zinc alloys



April 13, 1954 F. A. LOWEN HEIM `FLATING OF TIN-ZINC ALLOYS Filed Oct. l5, 1951 HAT/1V?" SPEE 9 9 Si IN VEN TDR. 'PEae'/ca/ 4 l 0mm/45141 Patented Apr. 13, 1954 PLATING OF TIN-ZINC ALLOYS Frederick A. Lowenheim, Plainield, N. J., assignor to Metal & Thermit Corporation, New York, N. Y., a corporation of New Jersey Application october 15, 1951, seriarNo. 251,312 1 Claim. (C1. coi- 43) The present invention relates to a process of electrodepositing alloys of tin and zinc.

It has been found that an alloy of tin and zinc, electrodeposited on metals such as steel, affords excellentprotection against corrosion. Tin and zinc can be deposited in all proportions from electrolyte solution but a most serviceable composition from the viewpoint of the protection afforded to the steel is around 80% tin, 20% zinc with a tolerance of about either way. The S50-20 tin-zinc alloy deposited has an appearance much like pure tin, that is a satiny white matte iinish and has many other -properties which make it a desirable replacement for cadmium. For example, (1) it has excellent corrosion resistance properties, (2) it is easily solderable even without flux, (3) the throwing and covering powers of the electrolyte bath are approximately equal to those of stannate tin, i. e. excellent, and therefore somewhat better than zinc or cadmium, and (4) it is economical to apply compared with cadmium.

Hitherto, the electrodeposition of the tin-zinc alloy has been accomplished from electrolytic solutions containing sodium stannate, sodium hydroxide, sodium zinc cyanide and free sodium cyanide. For replenishing the metal content of the electrolytic bath, alloy anodes of the same composition as the alloy to be deposited were employed. Y

With such a solution, the upper current density permitted with practicality is about 30 amperes per square foot. Attempts to increase this current density and therefore the rate of plating are nullied by a rapid falling 01T of cathode efliciency and a change in the proportions of tin and zinc in the deposit.

It has been found in accordance with the present invention that the use of potassium compounds instead of the sodium compounds in the electrolyte bath permits the electroplating operation to be performed at higher current densities up to 60 amperes per square foot and also makes the alloy deposition less dependent on the current densities. The deposition can thereby be controlled by the alloy anode and bath composition. The use of potassium compounds therefore permits the attainment of higher plating speeds and renders the process less critical in control.

The accompanying Figures 1-3 illustrate the advantages of the invention, wherein:

The curves of Figure 1 compare sodium and potassium compounds for current density versus cathode efficiency,

The curve of Figure 2 illustrates current density versus plating speed for potassium compounds, and

The curve of Figure 3 illustrates current density versus minutes to deposit for potassium compounds.

The solutions used in accordance with the present invention comprises basically potassium stannate plus a small amount of zinc cyanide, larger quantities of potassium cyanide and a small amount of free potassium hydroxide. For a deposit of an alloy containing between 15 and 25% of zinc andV between 75 and 85% of tin, the plating bath may comprise approximately between 32 and 45 grams per liter of tin, added as potassium stannate, between 3.5 and 9.0 grams per liter of zinc added as zinc cyanide, between 30 and 50 grams per liter of total potassium cyanide and between 5 and 11 grams per liter of ree potassium hydroxide. A typical formulation is as follows:

Example I Control Make-up, Limits (by grams/liter analysis),

grams/liter Potassium Stannate (corresponds to tin Z"i"fa" 430 37 A5 mc yan1 e correspon s o z c content) 5.o 3-5`60 Potassium Cyanide 19.0 30 45 Free Potassium Hydroxide 5 5.0- 8.0

It should be noted that potassium. cyanide by analysis differs from potassium cyanide actually added due to cyanide added as zinc cyanide. Thus analysis determines the total cyanide, which is free potassium cyanide plus the cyanide in the complex K2Zn(CN)4 or 2KCN.Zn(CN)2.

It should also be noted that the Wide control limits for the KCN are due partially to the permissible variation in zinc content because of the above relationship. The actual free cyanide should preferably vary only between 15 and 20 grams per liter.

The formulation in Example I is particularly useful for plating in still tanks. For barrel plating the following formulation may be employed:

Example II Control Make-up, Limits (by grams/liter analysis),

grams/liter Potassium Stannate (corresponds to tiu)- 32-40 Zinc Cyanide (corresponds to zinc) 7 9 Potassium Cyanide 15. 0 38-50 Free Potassium Hydroxide 101. 0 8-11 In both Examples I and II, the temperature of operation is 150 F., controlled to i5". Cathode current densities are inthe range of -75 amperes per square foot and preferably in the range of 30-50-aniperesper'square foot, `andanode current densities are about k--25 amperes'per square foot.

Anodes of the same composition as the deposit are employed, namely 80% tin and 20% zinc. The anodes must be lmed for satisfactory operation.

somewhat higher than the normaloperatingcurrent is impressed upon the anodes for-ashott time (usually less than a minute is suiicient), and then the current is outback to'the proper operating value. Formation of the lmacanbe noted by a rise in the voltage reading and/or a Y decrease in the ammeter reading. The anodes shouldjgas slightly, .an indication that .oxygen is being evolved and that the iilm is present.

.In the absence of the .anode lm, .the tin dissolves as stannous tin which will cause rough and spongy deposits and will also throw the bath composition out of balance.

A typical operation with potassium compounds Y in accordance with the present invention .gave the following results:

A curve following substantially this characteristie is indicated in Fig. 1.

.A typical operation, employing sodium compounds and in accordance with the prior art, gave the following results:v

\ Current Density Ilelmt Prlm `A curve-lfollowing:substantially this characteristc is indicated Fig. 1 for the purpose of-cornparison. Y

`The important advantageswhich are gained `by the use oiV potassium compounds insteadr of so- .current density 'tvlrhichtcansbe used :isgabout 30 amperes per square foot. Any current densities .beyond this level causes sudden drop in cathode In order to form ,the.lm, r.a :current Y eicieny beyond the limits of practical opera- 3tion. '@nfthe other hand,with potassium com- .RO-unds, eurrent...densities up to V amperes per .square iootv` caribe. employed at practically operableneciencies With potassium compounds in the electrolytic bath, the Icomposition of the alloy deposited is less-dependent on current density, asindicated in the comparative test data set forth above. Also,

withpotassium compounds, thecompositioniof the alloy 'deposited is less dependent. on theproportion of ingredients of the electrolytic bath.

InFig. 2; the Acurve indicates plating speed in Y ations j performed in the vpresence of potassium com-pounds' in the electrolyte'- bath.

While the invention has-been `describedl with particular reference to'specific embodiments, it vis-tobe understood that'it is not to be limited thereto' but is to be construed"broadlyand lrestricted solely by lthe lscope of 'the appended claim.

What is claimed is:

'A process/for the -electrolytic depositionof tin- Zinc @alloyv containing f 1522596l zinc andV A-"-'75'% tin which vcomprises subjectingl anarticle to be plated, yas a cathode,E to electrolysis inan aqueous platingI bath. containingv 32 to45-grams vper liter of tin in thelform-'of' potassium-stannate, V3.5 yto 9,0 grams per liter of zinc in the form'fof zinc cyanide, 3D1 toi-50 grams per' liter 'of 'totalfpotassiumfcyanidefand 5 to'lllrgrams peri liter of `free potassium l hydroxide, `While A`maintain-ingl acurrent density Aof between 30 f and50-1ampere's per square footp and' afbath temperature of between Al: andL155LF.

References cited in the fue of this vpatent UNITED sTATEsfPATENTs 

